Alzheimer’s disease is a neurological disorder. It is caused by physical changes in the brain, specifically the buildup of abnormal proteins that damage and kill nerve cells over time. An estimated 7.2 million Americans age 65 and older are living with Alzheimer’s dementia in 2025, making it the most common cause of dementia and one of the most studied neurological diseases in the world.
The question comes up because Alzheimer’s produces symptoms that can look psychiatric (depression, paranoia, hallucinations) alongside the cognitive decline most people associate with the disease. But these behavioral changes are driven by physical destruction of brain tissue. At its core, Alzheimer’s is a disease of the nervous system.
What Happens in the Brain
Two abnormal proteins define Alzheimer’s at the biological level: beta-amyloid and tau. Beta-amyloid is a fragment of a larger protein normally found in the brain. In Alzheimer’s, these fragments misfold and clump together into plaques that accumulate between nerve cells. The form known as amyloid-beta 42 is particularly prone to aggregating and has received the most attention in Alzheimer’s research, even though it’s present in lower concentrations than other forms.
Tau is a protein that normally helps stabilize the internal scaffolding of neurons. In Alzheimer’s, tau becomes hyperphosphorylated, meaning it picks up far more chemical tags than it should. A healthy tau molecule carries about two phosphate groups; tau extracted from Alzheimer’s brains carries roughly eight. This excess phosphorylation causes tau to misfold and tangle into twisted fibers called neurofibrillary tangles inside nerve cells. These tangles disrupt the cell’s transport system and eventually kill it.
Both amyloid plaques and tau tangles spread through a process called seeding, where misfolded proteins trigger neighboring normal proteins to misfold as well. This chain reaction is why the disease progresses steadily rather than staying confined to one area.
Which Brain Regions Are Affected First
Alzheimer’s does not damage the brain evenly. It follows a fairly predictable path. The earliest atrophy appears in the hippocampus, entorhinal cortex, and amygdala. The hippocampus is critical for forming new memories, which is why short-term memory loss is typically the first noticeable symptom. The entorhinal cortex acts as a relay station between the hippocampus and the rest of the brain, and the amygdala processes emotions.
As the disease advances, shrinkage spreads to the temporal lobe and parahippocampal region more broadly. In severe stages, multiple additional brain regions show measurably lower volume compared to earlier stages. The cortex, the brain’s outer layer responsible for language, reasoning, and social behavior, thins progressively. This widening pattern of damage explains why symptoms evolve from simple forgetfulness into problems with language, spatial awareness, judgment, and eventually basic physical functions.
Cognitive and Behavioral Symptoms
The symptoms most people recognize are cognitive: forgetting recent conversations, getting lost in familiar places, struggling to find words, losing track of dates. These reflect the neurological damage happening in memory and language centers.
But Alzheimer’s also produces a wide range of behavioral and psychological symptoms that can be confusing for families. These are sometimes called neuropsychiatric symptoms, and they shift as the disease progresses. Common ones include:
- Apathy: reduced motivation, social withdrawal, blunted emotional responses
- Depression: sadness, loss of interest, feelings of hopelessness
- Anxiety: persistent worry, tension
- Agitation: pacing, restlessness, repetitive behaviors
- Psychosis: paranoid delusions, visual or auditory hallucinations
- Sleep disorders: insomnia, excessive daytime sleep, restless leg movements
- Disinhibition: impulsive or socially inappropriate behavior
These symptoms are not separate psychiatric conditions. They emerge because the disease is physically destroying brain regions that regulate mood, impulse control, and perception. The specific behavioral profile changes as neurodegeneration reaches different areas.
How Doctors Confirm It’s Alzheimer’s
Because several types of dementia can look similar on the surface, the medical field has moved toward defining Alzheimer’s biologically rather than just by symptoms. The current research framework groups biomarkers into three categories, often abbreviated AT(N): amyloid (A), tau (T), and neurodegeneration (N).
Amyloid is measured through PET brain scans that light up amyloid deposits, or through spinal fluid tests showing low levels of amyloid-beta 42 (which drops in the fluid when it’s accumulating in the brain instead). Tau is measured via PET scans targeting tau tangles or spinal fluid tests showing elevated phosphorylated tau. Neurodegeneration is assessed through MRI scans showing brain shrinkage, PET scans measuring reduced brain metabolism, or spinal fluid markers of nerve cell damage.
For a diagnosis of Alzheimer’s disease specifically, biomarker evidence of both amyloid and tau must be present. This distinguishes it from other neurological conditions that cause dementia, like vascular dementia, which results from impaired blood flow to the brain rather than amyloid and tau accumulation.
How It Differs From Vascular Dementia
The distinction matters because treatment and prognosis differ. The traditional teaching was that Alzheimer’s has a slow, gradual onset while vascular dementia starts abruptly and worsens in a stepwise pattern, often after small strokes. In practice, this clean separation doesn’t always hold. Many older adults have both Alzheimer’s pathology and vascular damage simultaneously, and the clinical presentations can overlap considerably.
What reliably separates the two is the underlying biology. Alzheimer’s is defined by amyloid plaques and tau tangles. Vascular dementia is driven by damage to blood vessels in the brain. The biomarker framework described above is what allows clinicians to tell them apart in living patients, rather than relying solely on symptom patterns.
Genetic Risk Factors
The strongest known genetic risk factor for the common, late-onset form of Alzheimer’s is a variant of the APOE gene called APOE e4. Everyone inherits two copies of the APOE gene, one from each parent. Carrying one copy of the e4 variant doubles or triples the risk of developing Alzheimer’s. Carrying two copies raises the risk 8 to 12 times compared to people without the variant.
Having the e4 variant does not guarantee you’ll develop Alzheimer’s, and many people who get the disease don’t carry it at all. It’s a risk factor, not a cause. Rare, early-onset forms of Alzheimer’s (appearing before age 65) are linked to mutations in different genes that directly cause abnormal amyloid production, but these account for a small fraction of all cases.
Current Treatment Approaches
For decades, the only approved treatments for Alzheimer’s managed symptoms without addressing the underlying brain pathology. That changed with the approval of a new class of therapies: monoclonal antibodies designed to clear amyloid from the brain. Aducanumab, approved in 2021, was the first. It targets both soluble and insoluble forms of aggregated amyloid-beta. Lecanemab, approved in early 2023, primarily targets large clumps of soluble amyloid called protofibrils.
Both drugs work by binding to amyloid aggregates and flagging them for removal by the brain’s immune cells. They are approved for people in the early stages of the disease, when cognitive impairment is still mild. These are the first treatments that attempt to modify the disease process itself rather than just easing symptoms, though their clinical benefits have been modest and they carry risks including brain swelling and small bleeds.
The number of Americans with Alzheimer’s is projected to nearly double to 13.8 million by 2060 without major breakthroughs in prevention or treatment, underscoring why understanding this disease as a neurological condition with identifiable biological targets remains so important.